There are image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction machines including at least two of these functions, that include process units in which a development device, a toner cartridge, and photoreceptor are housed in a common unit casing as a modular unit removably installable in a main body of the image forming apparatus. Developer contained in process units is consumed in image development, and accordingly it is necessary to notify users when to replace the process unit.
Therefore, various types of detectors have been proposed to detect the amount of developer in a developer container in process units. For example, light transmission-type detectors including optical elements are used to detect the amount of developer. Light transmission-type developer amount detectors radiate light inside the developer container and determine the amount of developer therein based on the duration of time necessary for the light to transverse the developer container or the timing at which the light is detected.
For example, JP-2007-147764-A, JP-2005-345914-A, and JP-2007-219269-A propose light transmission-type developer amount detectors that include a light-emitting element, a light-receiving element, and first and second light guides. The light-emitting element and the light-receiving element are provided in the main body of the image forming apparatus. The first and second light guides are provided in the process unit and can be constructed of a prism, a mirror, or the like. The light emitted from the light-emitting element is guided by the first light guide into the developer container inside the process unit. Then, the second light guide guides the light out of the developer container to the light-receiving element.
When the amount of developer in the developer container is sufficient, the light is blocked by the developer, and the light-receiving element does not receive the light. By contrast, when the amount of developer in the developer container is reduced to or below a reference amount, the light can reach the light-receiving element. With the output from the light-receiving element at that time, it can be determined that the amount of developer has decreased below the reference amount.
Developer containers typically include a developer conveyance member such as a screw to transport the developer therein, thereby preventing local shortage of developer, even when images in which printing ratio is locally high are printed in succession, and a greater amount of developer is consumed in areas for forming such areas of high printing ratio. Detection accuracy of light transmission-type developer amount detectors, however, can be degraded in configurations in which developer is thus transported.
FIG. 9 is an end-on axial view of a development device that includes a developer conveyance screw and a developer amount detector according to a related art.
The development device shown in FIG. 9 includes a development housing 200 containing developer (toner) T, a screw 300 to transport the developer T, and first and second light guides 400 and 500. In this configuration, the light emitted from the light-emitting element is guided by the first light guide 400 into the development housing 200. At that time, if the toner T is present between the first and second light guides 400 and 500, the light is blocked. By contrast, if the toner T is not present between the first and second light guides 400 and 500, the light can pass through the second light guide 500 and reach the light-receiving element.
As the screw 300 rotates, the toner T is transported in the direction perpendicular to the surface of the paper on which FIG. 9 is drawn, and rotation of the screw 300 makes a surface J of the toner T slant as shown in FIG. 9. Although toner has a certain degree of fluidity, it is lower than that of liquid such as water. Accordingly, the surface J of the toner T does not keep the slant state but fluctuates even when the amount of the toner T does not change. If the slant surface J of the toner T positioned in a light transmission path L between the first light guide 400 and the second light guide 500 fluctuates as shown in FIG. 9 as the screw 300 rotates, the degree of light transmission varies, resulting in detection error. In other words, in the configuration in which the light transmission path L between the first light guide 400 and the second light guide 500 is perpendicular to the axis of the screw 300 as shown in FIG. 9, the degree of light transmission is susceptible to fluctuations in the surface of the toner T. Thus, it is difficult to attain a high degree of accuracy in detection of the toner amount.